1. Field of the Invention
The present invention relates to an automatic transmission.
2. Description of the Related Art
There are existing automatic transmissions capable of providing eight forward gears by using a first planetary gear mechanism for input, two planetary gear mechanisms for changing speed, and six engagement mechanisms (see, for example, Japanese Unexamined Patent Application Publication No. 2005-273768).
In the automatic transmission described in Japanese Unexamined Patent Application Publication No. 2005-273768, the first planetary gear mechanism for input is a double-pinion-type planetary gear mechanism including a first sun gear, a first ring gear, and a first carrier. The first carrier rotatably and revolvably supports a pair of first pinions that mesh with each other. One of the first pinions meshes with the first sun gear, and the other of the first pinions meshes with the first ring gear.
In the first planetary gear mechanism, the first sun gear is a fixed element fixed to a transmission case, the first carrier is an input element connected to an input shaft, and the first ring gear is an output element that reduces the rotation speed of the first carrier and outputs the rotation.
Each of the two planetary gear mechanisms for changing speed is a Ravigneaux-type planetary gear mechanism including a second sun gear, a third sun gear, a second ring gear that is integrated with a third ring gear, and a second carrier. The second carrier rotatably and revolvably supports a pair of second pinions that mesh with each other. One of the second pinions meshes with the second sun gear and the second ring gear, and the other of the second pinions meshes with the third sun gear.
In a velocity diagram, this Ravigneaux-type planetary gear mechanism has first to fourth rotation elements that are arranged in this order with distances, which correspond to the gear ratios, therebetween. The first rotation element is the second sun gear, the second rotation element is the second carrier integrated with the third carrier, the third rotation element is the second ring gear integrated with the third ring gear, and the fourth rotation element is the third sun gear.
The Ravigneaux-type planetary gear mechanism includes a first engagement mechanism that releasably connects the first ring gear, which is an output element of the first planetary gear mechanism, to the fourth rotation element, which is the third sun gear; a second engagement mechanism that releasably connects the input shaft to the second rotation element, which is the second carrier; a third engagement mechanism that releasably connects the first ring gear, which is an output element, to the first rotation element, which is the second sun gear; a fourth engagement mechanism that releasably connects the first carrier, which is an input element, to the first rotation element, which is the second sun gear; a fifth engagement mechanism that releasably fixes the first rotation element, which is the second sun gear, to the transmission case; and a sixth engagement mechanism that releasably fixes the second rotation element, which is the second carrier, to the transmission case.
With such a structure, a first gear is entered by engaging the first engagement mechanism and the sixth engagement mechanism, a second gear is entered by engaging the first engagement mechanism and the fifth engagement mechanism, a third gear is entered by engaging the first engagement mechanism and the third engagement mechanism, and the fourth gear is entered by engaging the first engagement mechanism and the fourth engagement mechanism.
A fifth gear is entered by engaging the first engagement mechanism and the second engagement mechanism, a sixth gear is entered by engaging the second engagement mechanism and the fourth engagement mechanism, a seventh gear is entered by engaging the second engagement mechanism and the third engagement mechanism, and an eighth gear is entered by engaging the second engagement mechanism and the fifth engagement mechanism. The automatic transmission may have seven forward gears by omitting the first gear or the eighth gear.
In the existing example described above, two engagement mechanisms are engaged in each gear. Therefore, large friction loss is caused by drag of the remaining four engagement mechanisms that are not engaged, so that the efficiency of the transmission is reduced.